Semiconductor device including a capacitance

ABSTRACT

It is an object to obtain a semiconductor device including a capacitance having a great Q-value. In an SOI substrate comprising a support substrate ( 165 ), a buried oxide film ( 166 ) and an SOI layer ( 171 ), an isolating oxide film  167  ( 167   a  to  167   c ) is selectively formed in an upper layer portion of the SOI layer ( 171 ) with a part of the SOI layer ( 171 ) remaining as a P −  well region ( 169 ). Consequently, an isolation (partial isolation) structure is obtained. An N +  diffusion region ( 168 ) is formed in the SOI layer ( 171 ) between the isolating oxide films ( 167   a ) and ( 167   b ) and a P +  diffusion region ( 170 ) is formed in the SOI layer ( 171 ) between the isolating oxide films ( 167   b ) and ( 167   c ). Consequently, there is obtained a junction type variable capacitance (C 23 ) having a PN junction surface of the P −  well region ( 169 ) provided under the isolating oxide film ( 167   b ) and the N +  diffusion region ( 168 ).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device having aninsulated gate type transistor and an insulated gate type capacitanceand a method of manufacturing the semiconductor device.

2. Description of the Background Art

In a transistor having a gate length of a subquarter micron or less, apocket injecting process for forming a pocket region is executed inorder to suppress a short channel effect. The pocket injection is alsoreferred to as NUDC (Non Uniformly Doped Channel).

FIG. 52 is a sectional view showing the pocket injecting process. Asshown in FIG. 52, in the formation of a CMOS transistor, an NMOSformation region A11 and a PMOS formation region A12 are isolated fromeach other through an isolating film 102 provided in an upper layerportion of a semiconductor substrate 101.

In the NMOS formation region A11, a gate oxide film 112 and a gateelectrode 113 are formed on a surface of a P well region 111 and aP-type impurity ion 103 is implanted and diffused by using the gateelectrode 113 as a mask. Consequently, a P-type impurity implantationregion 119 to be a pocket region of an NMOS transistor is formed.

In the PMOS formation region A12, similarly, a gate oxide film 122 and agate electrode 123 are formed on a surface of an N well region 121 andan N-type impurity ion 104 is implanted and diffused by using the gateelectrode 123 as a mask. Consequently, an N-type impurity implantationregion 129 to be a pocket region of a PMOS transistor is formed.

More specifically, in the pocket injecting process, an impurity of thesame conductivity type as that of a channel region of each MOStransistor is implanted into each of the NMOS formation region All andthe PMOS formation region A12. In the pocket injecting process, thedistribution of an impurity in a direction of a channel length becomesnonuniform and an effective channel impurity concentration is increasedwhen a gate length becomes smaller. As a result, it is possible tosuppress the short channel effect.

FIG. 53 is a sectional view showing a state in which a CMOS transistoris finished after the pocket injecting process.

As shown in FIG. 53, in the NMOS formation region A11, N⁺ source-drainregions 114 and 114 are formed to interpose therebetween a channelregion provided under the gate electrode 113 and tip regions opposed toeach other between the N⁺ source-drain regions 114 and 114 are extensionportions 114 e, respectively.

In a vicinal region of the extension portion 114 e, the P-type impurityimplantation region 119 remains as a P⁻ pocket region 117 from theextension portion 114 e to a part of the channel region. Moreover, sidewalls 116 and 116 are formed on both side surfaces of the gate electrode113, respectively.

Thus, an NMOS transistor Q11 is formed by the gate oxide film 112, thegate electrode 113, the N⁺ source-drain region 114, the side wall 116and the P⁻ pocket region 117.

In the PMOS formation region A12, P⁺ source-drain regions 124 and 124are formed to interpose therebetween a channel region provided under thegate electrode 123 and tip regions opposed to each other between the P⁺source-drain regions 124 and 124 are extension portions 124 e,respectively.

In a vicinal region of the extension portion 124 e, the N-type impurityimplantation region 129 remains as an N⁻ pocket region 127 from theextension portion 124 e to a part of the channel region. Moreover, sidewalls 126 and 126 are formed on both side surfaces of the gate electrode123, respectively.

Thus, a PMOS transistor Q12 is formed by the gate oxide film 122, thegate electrode 123, the P⁺ source-drain region 124, the side wall 126and the N⁻ pocket region 127.

On the other hand, in a high-frequency analog circuit or a high-speeddigital circuit, it is necessary to manufacture an LC type VCO (VoltageControlled Oscillator) by using an inductor (L) and a variablecapacitance (C).

In the case in which the variable capacitance to be an insulated gatetype capacitance which has a small loss is to be obtained by utilizingthe structure of the MOS transistor, it is necessary to generate anaccumulation type variable capacitance in which impurities in asubstrate (a body region) and a fetch electrode portion have the sameconductivity type.

FIG. 54 is a sectional view showing a structure of the accumulation typevariable capacitance. As shown in FIG. 54, in the formation of theaccumulation type variable capacitance, a P-type variable capacitanceformation region A13 and an N-type variable capacitance formation regionA14 are isolated from each other through an isolating film 102 providedin an upper layer portion of a semiconductor substrate 101.

In the P-type variable capacitance formation region A13, P⁺ fetchelectrode regions 134 and 134 are formed to interpose therebetween achannel region provided under a gate electrode 133 and tip regionsopposed to each other between the P⁺ fetch electrode regions 134 and 134are extension portions 134 e, respectively.

In a vicinal region of the extension portion 134 e, an N⁻ pocket region137 is formed from the extension portion 134 e to a part of the channelregion. Moreover, side walls 136 and 136 are formed on both sidesurfaces of the gate electrode 133, respectively.

Thus, a P-type variable capacitance C11 is formed by a gate oxide film132, the gate electrode 133, the P⁺ fetch electrode region 134, the sidewall 136 and the N⁻ pocket region 137. In other words, the P-typevariable capacitance C11 acts as an insulated gate type capacitance inwhich the P⁺ fetch electrode region 134 is set to one of electrodes, thegate electrode 133 is set to the other electrode and the gate oxide film132 is set to an interelectrode insulating film.

In the N-type variable capacitance formation region A14, N⁺ fetchelectrode regions 144 and 144 are formed to interpose therebetween achannel region provided under a gate electrode 143 and tip regionsopposed to each other between the N⁺ fetch electrode regions 144 and 144are extension portions 144 e, respectively.

In a vicinal region of the extension portion 144 e, a P⁻ pocket region147 is formed from the extension portion 144 e to a part of the channelregion. Moreover, side walls 146 and 146 are formed on both sidesurfaces of the gate electrode 143, respectively.

Thus, an N-type variable capacitance C12 is formed by a gate oxide film142, the gate electrode 143, the N⁺ fetch electrode region 144, the sidewall 146 and the P⁻ pocket region 147. In other words, the N-typevariable capacitance C12 acts as an insulated gate type capacitance inwhich the N⁺ fetch electrode region 144 is set to one of electrodes, thegate electrode 143 is set to the other electrode and the gate oxide film142 is set to an interelectrode insulating film.

FIGS. 55 and 56 are views illustrating a degree of a change in acapacitance value of the N-type variable capacitance C12. In the case inwhich a gate voltage VG to be applied to the gate electrode 143 is lowerthan 0 V, a depletion layer 148 is extended downward in an N well region121 provided under the gate electrode 143 as shown in FIG. 55 so that acapacitance value of the N-type variable capacitance C12 is decreased.On the other hand, in the case in which the gate electrode VG is higherthan 0 V, the depletion layer 148 is reduced in the N well region 121provided under the gate electrode 143 as shown in FIG. 56 so that thecapacitance value of the N-type variable capacitance C12 is increased.Thus, it is possible to variably set the capacitance value of the N-typevariable capacitance C12 based on the gate voltage VG to be applied tothe gate electrode 143. Also in the P-type variable capacitance C11,similarly, it is possible to change the capacitance value based on thegate voltage to be applied to the gate electrode 133.

However, when the pocket injecting process shown in FIG. 52 is executedin order to enhance a short channel characteristic (to suppress theshort channel effect), a pocket region of a reverse conductivity type tothat of the body region is formed with the accumulation type variablecapacitance in a fetch electrode region and the body region to be aregion of the semiconductor substrate 101 which is provided just belowthe gate electrode. Therefore, there has been a problem in that a seriesresistance is increased.

FIG. 57 is a circuit diagram showing an equivalent circuit of thevariable capacitance in FIG. 54. As shown in FIG. 57, the variablecapacitance is equivalently represented by a series connection of acapacitance component C10 and a resistance component R10.

On the other hand, an index representing an electrical characteristic ofthe variable capacitance includes a Q—factor (Q-value). The Q-value isexpressed in the following equation (1), wherein Q represents a Q-value,ω represents an angular frequency, C represents a capacitance value ofthe capacitance component C10 and R represents a resistance value of theresistance component R10. $\begin{matrix}{{\left\lbrack {{Equation}\quad 1} \right\rbrack\quad\quad{Q = \frac{1}{\omega\quad C\quad R}}}\quad} & (1)\end{matrix}$

When the Q-value is increased, an energy efficiency of the capacitanceis enhanced. There has been a problem in that the resistance value R ofthe resistance component R10 is increased by the presence of the pocketregion so that the Q-value is decreased in accordance with the equation(1). In addition, there has been a problem in that an insulated gatetype capacitance generally has a small Q-value.

SUMMARY OF THE INVENTION

It is an object of the present invention to obtain a semiconductordevice comprising a capacitance having a great Q-value.

According to a first aspect of the present invention, a semiconductordevice includes a fixed capacitance and a variable capacitance. Thefixed capacitance is formed in a semiconductor substrate and has acapacitance value fixed, and the variable capacitance is formed in thesemiconductor substrate and is capable of variably controlling acapacitance value. The fixed capacitance and the variable capacitanceare connected to each other in parallel.

A capacitance having a great Q-value is used for at least one of thefixed capacitance and the variable capacitance. Consequently, it ispossible to enhance the Q-value of a composite capacitance of the fixedcapacitance and the variable capacitance.

According to a second aspect of the present invention, a semiconductordevice has a junction capacitance formed in an SOI layer of an SOIsubstrate constituted by a substrate, at least a surface of which isinsulative, and the SOI layer of a first conductivity type provided onthe surface of the substrate. The junction capacitance includes firstand second junction semiconductor regions. The first junctionsemiconductor region is of a second conductivity type and is formed onthe SOI layer, and the second semiconductor region is of a firstconductivity type and is formed on the SOI layer. The first and secondsemiconductor regions have a PN junction portion.

The junction capacitance is formed on the SOI substrate. Consequently,it is possible to obtain the junction capacitance which is lessinfluenced by the parasitic capacitance.

The junction capacitance further includes a first semiconductor regionof the second conductivity type, a second semiconductor region of thefirst conductivity type and an isolating region. The first semiconductorregion is formed on the SOI layer. The second semiconductor region isformed on the SOI layer independently of the first semiconductor region.The isolating region is provided in an upper layer portion of the SOIlayer and serves to isolate the first and second semiconductor regionsfrom each other.

The isolating region includes a partial isolating region constituted bya partial insulating region provided in an upper layer portion of theSOI layer and an isolating semiconductor region of the firstconductivity type which is a part of the SOI layer present in a lowerlayer portion. The first junction semiconductor region includes thefirst semiconductor region. The second junction semiconductor regionincludes the second semiconductor region and the isolating semiconductorregion. The isolating semiconductor region has a PN junction portiontogether with the first semiconductor region.

According to a third aspect of the present invention, a semiconductordevice has a junction capacitance formed in an SOI layer of an SOIsubstrate constituted by a substrate, at least a surface of which isinsulative, and the SOI layer of a first conductivity type provided onthe surface of the substrate. The junction capacitance includes firstand second junction semiconductor regions. The first junctionsemiconductor region is of a second conductivity type and is formed onthe SOI layer, and the second semiconductor region is of a firstconductivity type and is formed on the SOI layer. The first and secondsemiconductor regions have a PN junction portion.

The junction capacitance is formed on the SOI substrate. Consequently,it is possible to obtain the junction capacitance which is lessinfluenced by the parasitic capacitance.

For a capacitance of the first conductivity type, the junctioncapacitance includes a body region, a first semiconductor region of thesecond conductivity type and a second semiconductor region of the firstconductivity type. The body region for a capacitance is formed in asurface of the SOI layer. The first and second semiconductor regions areformed to interpose therebetween the body region for a capacitance, thesecond semiconductor region having an impurity concentration to be sethigher than that in the body region for a capacitance.

The first junction semiconductor region includes the first semiconductorregion. The second junction semiconductor region includes the secondsemiconductor region and the body region for a capacitance. The firstsemiconductor region has a PN junction portion together with the bodyregion for a capacitance.

According to a fourth aspect of the present invention, a semiconductordevice includes an insulated gate type capacitance formed in asemiconductor substrate. The insulated gate type capacitance includes agate insulating film for a capacitance, a gate electrode for acapacitance and extraction electrode regions. The gate insulating filmfor a capacitance is selectively formed on the semiconductor substrate,the gate electrode for a capacitance is formed on the gate insulatingfilm for a capacitance, and the extraction electrode regions are formedto interpose therebetween a body region for a capacitance which isprovided under the gate electrode for a capacitance in a surface of thesemiconductor substrate. The gate electrode for a capacitance has, onone of ends and the other end, first and second contact pad portionscapable of being electrically connected to an external wiring.

The electrical connection to the external wiring can be carried outthrough the first and second contact pad portions. Consequently, it ispossible to reduce a resistance value attendant on the gate electrodefor a capacitance.

According to a fifth aspect of the present invention, a semiconductordevice includes an insulated gate type capacitance formed in asemiconductor substrate. The insulated gate type capacitance includes agate insulating film for a capacitance, a gate electrode for acapacitance and extraction electrode regions. The gate insulating filmfor a capacitance is selectively formed on the semiconductor substrate,the gate electrode for a capacitance is formed on the gate insulatingfilm for a capacitance, and the extraction electrode regions are formedto interpose therebetween a body region for a capacitance which isprovided under the gate electrode for a capacitance in a surface of thesemiconductor substrate. Each of the extraction electrode regions have ahollow portion in a central region seen on a plane, and the gateelectrode for a capacitance has a plurality of partial gate portionsextended from the hollow portion radially as seen on a plane.

The gate electrode for a capacitance is constituted by a plurality ofpartial gate portions. Consequently, a length of each partial gateportion is defined as a gate width. Thus, it is possible to reduce theresistance value of the gate electrode.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing a structure of a semiconductor devicehaving a MOS transistor and a variable capacitance according to a firstembodiment of the present invention,

FIGS. 2 to 7 are sectional views showing a method of manufacturing thesemiconductor device according to the first embodiment,

FIG. 8 is a sectional view showing a structure of a semiconductor deviceaccording to a second embodiment of the present invention,

FIG. 9 is a view illustrating the influence of a high frequency currentof an N-type variable capacitance according to the second embodiment,

FIG. 10 is a view illustrating the influence of a high frequency currentof an N-type variable capacitance having a pocket region formed in anSOI substrate,

FIG. 11 is a sectional view showing a structure of a semiconductordevice according to a third embodiment of the present invention,

FIGS. 12 to 15 are sectional views showing a method of manufacturing asemiconductor device according to a fourth embodiment,

FIG. 16 is a sectional view showing a structure of a semiconductordevice according to a fifth embodiment of the present invention,

FIG. 17 is a sectional view showing a structure of a semiconductordevice according to a sixth embodiment of the present invention,

FIG. 18 is a diagram illustrating a layout structure of a semiconductordevice according to a seventh embodiment of the present invention,

FIG. 19 is a sectional view showing a structure of a high voltagetransistor formation region and a variable capacitance formation regionin the semiconductor device according to the seventh embodiment,

FIG. 20 is a sectional view showing a structure of a semiconductordevice according to an eighth embodiment of the present invention,

FIG. 21 is a sectional view showing a part of a method of manufacturinga semiconductor device having a MOS transistor and a variablecapacitance according to a ninth embodiment of the present invention,

FIGS. 22 to 26 are sectional views showing a method of manufacturing anN-type variable capacitance in a semiconductor device according to atenth embodiment,

FIG. 27 is a sectional view showing a structure of a semiconductordevice according to an eleventh embodiment of the present invention,

FIG. 28 is a sectional view showing a structure of a first mode of asemiconductor device according to a twelfth embodiment of the presentinvention,

FIG. 29 is a sectional view showing a structure of a second mode of thesemiconductor device according to the twelfth embodiment of the presentinvention,

FIG. 30 is a sectional view showing a structure of a first mode of asemiconductor device according to a thirteenth embodiment of the presentinvention,

FIGS. 31 to 35 are sectional views showing a method of forming a poroussilicon layer,

FIG. 36 is a sectional view showing a structure of a junction typevariable capacitance according to a fourteenth embodiment,

FIG. 37 is a view illustrating an equivalent circuit of a parallelconnection type variable capacitance according to the fourteenthembodiment,

FIG. 38 is a sectional view showing a structure of a junction typevariable capacitance in which an isolation is carried out,

FIG. 39 is a plan view showing a planar structure of the junction typevariable capacitance in FIG. 38,

FIG. 40 is a sectional view showing a structure of a junction typevariable capacitance according to a fifteenth embodiment of the presentinvention,

FIG. 41 is a sectional view showing a structure of a first mode of ajunction type variable capacitance according to a sixteenth embodimentof the present invention,

FIG. 42 is a sectional view showing a structure of a second mode of thejunction type variable capacitance according to the sixteenth embodimentof the present invention,

FIG. 43 is a sectional view showing a structure of a third mode of thejunction type variable capacitance according to the sixteenth embodimentof the present invention,

FIG. 44 is a sectional view showing a structure of a fourth mode of thejunction type variable capacitance according to the sixteenth embodimentof the present invention,

FIG. 45 is a sectional view showing a structure of an MIM typecapacitance according to a seventeenth embodiment of the presentinvention,

FIG. 46 is a circuit diagram showing an example of a VCO,

FIG. 47 is a circuit diagram showing a part of a circuit structure of aVCO according to an eighteenth embodiment of the present invention,

FIG. 48 is a plan view showing a gate contact structure of an insulatedgate type capacitance according to a nineteenth embodiment of thepresent invention,

FIG. 49 is a plan view showing a first mode of a gate contact structureof an insulated gate type capacitance according to a twentiethembodiment of the present invention,

FIG. 50 is a plan view showing a second mode of the gate contactstructure of the insulated gate type capacitance according to thetwentieth embodiment of the present invention,

FIG. 51 is a plan view showing a third mode of the gate contactstructure of the insulated gate type capacitance according to thetwentieth embodiment of the present invention,

FIGS. 52 and 53 are sectional views showing a method of manufacturing aMOS transistor having a pocket region according to a conventional art,

FIG. 54 is a sectional view showing a structure of a variablecapacitance,

FIGS. 55 and 56 are views illustrating an operation for setting acapacitance value of the variable capacitance, and

FIG. 57 is a circuit diagram showing an equivalent circuit of thevariable capacitance illustrated in FIG. 54.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

<First Embodiment>

FIG. 1 is a sectional view showing a structure of a semiconductor devicehaving a MOS transistor and a variable capacitance according to a firstembodiment of the present invention.

As shown in FIG. 1, an NMOS transistor Q1, a PMOS transistor Q2, anN-type variable capacitance C1 and a P-type variable capacitance C2 areformed in an NMOS formation region A1, a PMOS formation region A2, anN-type variable capacitance formation region A3 and a P-type variablecapacitance formation region A4, respectively. Each of the formationregions A1 to A4 is isolated by an isolating film (not shown) or thelike. Moreover, well regions 11, 21, 31 and 41 to be body regions areformed in the formation regions A1, A2, A3 and A4, respectively.

In the NMOS formation region A1, a gate oxide film 12 is selectivelyformed on a surface of the P well region 11 and an N⁺-type gateelectrode 13 is formed on the gate oxide film 12. N⁺ source-drainregions 14 and 14 are formed to interpose therebetween a channel regionto be a surface region of the P well region 11 provided under the gateelectrode 13, and tip regions protruded and opposed to each otherbetween the N⁺ source-drain regions 14 and 14 are extension portions 14e, respectively.

In a vicinal region of the extension portion 14 e, P⁻ pocket regions 17are formed from the extension portion 14 e to a part of the channelregion. Moreover, side walls 16 and 16 are formed on both side surfacesof the gate electrode 13, respectively. Furthermore, a silicide region14 s and a silicide region 13 s are formed in a surface of the N⁺source-drain regions 14 and an upper layer portion of the gate electrode13, respectively.

Thus, the NMOS transistor Q1 is formed by the gate oxide film 12, thegate electrode 13, the N⁺ source-drain regions 14, the side wall 16 andthe P⁻ pocket regions 17.

In the PMOS formation region A2, a gate oxide film 22 is selectivelyformed on a surface of the N well region 21 and a P⁺-type gate electrode23 is formed on the gate oxide film 22. P⁺ source-drain regions 24 and24 are formed to interpose therebetween a channel region provided underthe gate electrode 23, and tip regions protruded and opposed to eachother between the P⁺ source-drain regions 24 and 24 are extensionportions 24 e, respectively.

In a vicinal region of the extension portion 24 e, N⁻ pocket regions 27are formed from the extension portion 24 e to a part of the channelregion. Moreover, side walls 26 and 26 are formed on both side surfacesof the gate electrode 23, respectively. Furthermore, a silicide region24 s and a silicide region 23 s are formed in a surface of the P⁺source-drain regions 24 and an upper layer portion of the gate electrode23, respectively.

Thus, a PMOS transistor Q2 is formed by the gate oxide film 22, the gateelectrode 23, the P⁺ source-drain regions 24, the side wall 26 and theN⁻ pocket regions 27.

In the N-type variable capacitance formation region A3, a gate oxidefilm 32 is selectively formed on a surface of the N well region 31 andan N⁺-type gate electrode 33 is formed on the gate oxide film 32. N⁺extraction electrode regions 34 and 34 are formed to interposetherebetween a body surface region to be a surface of the N well region31 provided under the gate electrode 33. Tip regions protruded andopposed to each other between the N⁺ extraction electrode regions 34 and34 are extension portions 34 e, respectively.

Moreover, side walls 36 and 36 are formed on both side surfaces of thegate electrode 33, respectively. Furthermore, a silicide region 34 s anda silicide region 33 s are formed in a surface of the N⁺ extractionelectrode regions 34 and an upper layer portion of the gate electrode33, respectively.

Thus, an N-type (N⁺ gate/N⁻ body type) variable capacitance C1 is formedby the gate oxide film 32, the gate electrode 33, the N⁺ extractionelectrode regions 34 and the side wall 36. More specifically, the N-typevariable capacitance C1 becomes an insulated gate type capacitance inwhich the N⁺ extraction electrode regions 34 are set to one ofelectrodes, the gate electrode 33 is set to the other electrode and thegate oxide film 32 is set to an interelectrode insulating film. By agate voltage to be applied to the gate electrode 33, it is possible tovariably set a capacitance value by changing the extension of adepletion layer in the N well region 31 provided under the gateelectrode 33.

In the P-type variable capacitance formation region A4, a gate oxidefilm 42 is selectively formed on a surface of the P well region 41 and aP⁺-type gate electrode 43 is formed on the gate oxide film 42. P⁺extraction electrode regions 44 and 44 are formed to interposetherebetween a body surface region provided under the gate electrode 43.Tip regions protruded and opposed to each other between the P⁺extraction electrode regions 44 and 44 are extension portions 44 e,respectively.

Moreover, side walls 46 and 46 are formed on both side surfaces of thegate electrode 43, respectively. Furthermore, a silicide region 44 s anda silicide region 43 s are formed in a surface of the P⁺ extractionelectrode region 44 and an upper layer portion of the gate electrode 43,respectively.

Thus, a P-type (P⁺ gate/P⁻ body type) variable capacitance C2 is formedby the gate oxide film 42, the gate electrode 43, the P⁺ extractionelectrode regions 44 and the side wall 46. More specifically, the P-typevariable capacitance C2 becomes an insulated gate type capacitance inwhich the P⁺ extraction electrode regions 44 are set to one ofelectrodes, the gate electrode 43 is set to the other electrode and thegate oxide film 42 is set to an interelectrode insulating film. By agate voltage to be applied to the gate electrode 43, it is possible tovariably set a capacitance value by changing the extension of adepletion layer in the P well region 41 provided under the gateelectrode 43.

As described above, in the semiconductor device according to the firstembodiment, the pocket regions are present in the MOS transistor.Therefore, it is possible to obtain a MOS transistor in which a shortchannel effect is suppressed.

On the other hand, pocket regions (regions of a reverse conductivitytype which are adjacent to extraction electrode regions) are not presentin the variable capacitance. More specifically, the region of a reverseconductivity type to that of the extraction electrode regions is notpresent at all in a region provided in the vicinity of the extractionelectrode regions of the variable capacitance. Therefore, a seriesresistance is low and a Q-value is not deteriorated.

As the semiconductor device according to the first embodiment, thus, itis possible to obtain a semiconductor device comprising a MOS transistorsuppressing a short channel effect and a variable capacitance in which aseries resistance is low and a Q-value is not deteriorated.

FIGS. 2 to 7 are sectional views showing a method of manufacturing thesemiconductor device according to the first embodiment. A procedure formanufacturing the semiconductor device according to the first embodimentwill be described below with reference to these drawings.

As shown in FIG. 2, first of all, a P well region 11, an N well region21, an N well region 31 and a P well region 41 which are to be bodyregions are formed, by an existing method, in an NMOS formation regionA1, a PMOS formation region A2, an N-type variable capacitance formationregion A3 and a P-type variable capacitance formation region A4 whichare isolated from each other. Then, a gate oxide film 12 and an N⁺-typegate electrode 13 are selectively formed on a surface of the P wellregion 11, a gate oxide film 22 and a P⁺-type gate electrode 23 areselectively formed on a surface of the N well region 21, a gate oxidefilm 32 and an N⁺-type gate electrode 33 are selectively formed on asurface of the N well region 31, and a gate oxide film 42 and a P⁺-typegate electrode 43 are selectively formed on a surface of the P wellregion 41.

As shown in FIG. 3, a resist 51 is formed on a region other than theNMOS formation region A1, and a P-type impurity ion 61 and an N-typeimpurity ion 62 are then implanted and diffused sequentially into onlythe NMOS formation region A1 by varying an implantation energy with thegate electrode 13 to be a mask. Thus, a P⁻ diffusion region 19 and an N⁻extension region 18 are formed, respectively.

As a specific example of the implantation of the N-type impurity ion 62,it can be proposed that an arsenic ion is implanted at an implantationenergy of 3 to 20 keV, a dose of 1×10¹⁴ to 1×10¹⁵/cm², and animplantation angle of 0 degree.

As a specific example of the implantation of the P-type impurity ion 61,moreover, it can be proposed that a boron ion is implanted at animplantation energy of 10 to 20 keV, a dose of 1×10¹³ to 3×10¹³/cm², andan implantation angle of 0 to 45 degrees.

As shown in FIG. 4, subsequently, the resist 51 is removed and a resist52 is then formed on a region other than the N-type variable capacitanceformation region A3 and an N-type impurity ion 63 is thereafterimplanted into only the N-type variable capacitance formation region A3with the same contents as those of the implantation of the N-typeimpurity ion 62, for example. Consequently, an N⁻ extension region 38 isformed. In this case, it can also be proposed that an N-type impurityion is implanted on the same conditions as those of an N-type impurityion 64 which will be described below, thereby further forming N⁻ pocketregions.

As shown in FIG. 5, next, the resist 52 is removed and a resist 53 isthen formed on a region other than the PMOS formation region A2, and theN-type impurity ion 64 and a P-type impurity ion 65 are then implantedand diffused sequentially into only the PMOS formation region A2 byvarying an implantation energy with the gate electrode 23 to be a mask.Thus, an N diffusion region 29 and a P⁻ extension region 28 are formed.

As a specific example of the implantation of the P-type impurity ion 65,moreover, it can be proposed that a BF₂ ion is implanted at animplantation energy of 3 to 10 keV, a dose of 1×10¹⁴ to 1×10¹⁵/cm², andan implantation angle of 0 degree.

As a specific example of the implantation of the N-type impurity ion 64,furthermore, it can be proposed that an arsenic ion is implanted at animplantation energy of 50 to 150 keV, a dose of 1×10¹³ to 3×10¹³/cm²,and an implantation angle of 0 to 45 degrees.

As shown in FIG. 6, subsequently, the resist 53 is removed and a resist54 is then formed on a region other than the P-type variable capacitanceformation region A4 and a P-type impurity ion 66 is thereafter implantedinto only the P-type variable capacitance formation region A4 with thesame contents as those of the implantation of the P-type impurity ion65, for example. Consequently, a P⁻ extension region 48 is formed. Inthis case, it can also be proposed that a P-type impurity ion isimplanted on the same conditions as those of the P-type impurity ion 61to further form P⁻ pocket regions.

When the resist 54 is removed as shown in FIG. 7, it is possible toobtain such a structure that a P⁻ diffusion region 19 and an N⁻diffusion region 29 which are to be pocket regions are present in onlythe MOS transistor formation regions A1 and A2 and a diffusion region tobe the pocket regions is not present in the variable capacitanceformation regions A3 and A4.

Subsequently, the structure shown in FIG. 1 can be obtained by using anexisting method of forming a MOS transistor and a variable capacitance.In the structure shown in FIG. 1, a side wall is formed and source-drainregions (extraction electrode regions) are then formed, and furthermore,the inside of a surface of the source-drain regions (extractionelectrode regions) and an upper layer portion of the gate electrode aresilicided by a self-align silicide (salicide) process, thereby forming asilicide region and reducing a resistance.

As a specific example of the formation of the N⁺ source-drain regions 14of the NMOS transistor Q1, it can be proposed that an arsenic ion isimplanted at an implantation energy of 20 to 70 keV, a dose of 1×10¹⁵ to1×10¹⁶/cm², and an implantation angle of 0 to 30 degrees.

As a specific example of the formation of the P⁺ source-drain regions 24of the PMOS transistor Q2, moreover, it can be proposed that a BF₂ ionis implanted at an implantation energy of 10 to 30 keV, a dose of 1×10¹⁵to 1×10¹⁶/cm², and an implantation angle of 0 to 30 degrees.

For example, CoSi₂, TiSi₂, NiSi₂ or the like is used for the silicide.

While both of the N- and P-type variable capacitances are formed in thepresent embodiment, only one of the variable capacitances may be formed.It is preferable that a variable capacitance of such a type as to beconvenient for a circuit should be formed, and the N-type variablecapacitance has a low resistance value of a series resistance componentof a body portion and a great Q-value. In this respect, the N-typevariable capacitance is more excellent.

<Second Embodiment>

FIG. 8 is a sectional view showing a structure of a semiconductor devicehaving a MOS transistor and a variable capacitance according to a secondembodiment of the present invention.

As shown in FIG. 8, a buried oxide film 4 is formed on a supportsubstrate 3, and an SOI layer 5 formed on the buried oxide film 4 isisolated into an NMOS formation region A1, a PMOS formation region A2,an N-type variable capacitance formation region A3 and a P-type variablecapacitance formation region A4 through an isolating film (not shown) orthe like.

An NMOS transistor Q1, a PMOS transistor Q2, an N-type variablecapacitance C1 and a P-type variable capacitance C2 which have the samestructures as those of the first embodiment are formed in the NMOSformation region A1, the PMOS formation region A2, the N-type variablecapacitance formation region A3 and the P-type variable capacitanceformation region A4, respectively.

In the semiconductor device according to the second embodiment, thus,the MOS transistors Q1 and Q2 and the variable capacitances C1 and C2which are similar to those of the first embodiment are formed on the SOIsubstrate (the support substrate 3, the buried oxide film 4 and the SOIlayer 5). Accordingly, the structure and manufacturing method are thesame as those in the first embodiment except that a bulk substrate isreplaced with the SOI substrate.

FIG. 9 is a view illustrating the influence of a high frequency currentof the N-type variable capacitance according to the second embodiment.As shown in FIG. 9, a high frequency current path CP1 mainly flows inthe N-type variable capacitance C1 between N⁺ extraction electroderegions 34 in the vicinity of a gate oxide film 32 and a gate electrode33. Therefore, a variable capacitance characteristic is not greatlydeteriorated.

FIG. 10 is a view illustrating the influence of a high frequency currentof an N-type variable capacitance having pocket regions formed in theSOI substrate. As shown in FIG. 10, a thickness of the SOI layer 5 issmaller than that of the bulk substrate so that a part (shown in adotted line) of a high frequency current path CP2 flowing in an N-typevariable capacitance C1P is made invalid and a series resistance isincreased. Consequently, a degree of deterioration is great.

Thus, when P⁻ pocket regions 37 are present, the bad influence of thehigh frequency current path CP2 is increased. For the SOI structure,therefore, the structure according to the second embodiment in which thepocket regions are not provided in the variable capacitance is veryeffective.

<Third Embodiment>

FIG. 11 is a sectional view showing a structure of a semiconductordevice having a MOS transistor and a variable capacitance according to athird embodiment of the present invention.

As shown in FIG. 11, variable capacitances C3 and C4 of an inversiontype are provided in place of the variable capacitances C1 and C2 of anaccumulation type. More specifically, the structure according to thethird embodiment is different from the structure according to the firstembodiment shown in FIG. 1 in that a P well region 30 is provided inplace of the N well region 31 and an N well region 40 is provided inplace of the P well region 41.

More specifically, the N-type (N⁺ gate/P⁻ body/N⁺S/D type) variablecapacitance C3 and the P-type (P⁺ gate/N⁻ body/P⁺S/D type) variablecapacitance C4 have structures equivalent to the structures of the NMOStransistor and the PMOS transistor, and are different from the NMOStransistor Q1 and the PMOS transistor Q2 in that pocket regionscorresponding to the pocket regions 17 and 27 are not provided.

In the variable capacitance having the inversion type structure, thepocket regions and the body region (the P well region 30, the N wellregion 40) have the same conductivity types even if the pocket regionsare provided. Differently from the variable capacitance of theaccumulation type, therefore, a resistance value of a series resistancecomponent of the body region is not increased.

However, when the pocket regions are formed, an impurity concentrationdistribution in a direction of a channel length becomes nonuniform.Therefore, there is a problem in that a distribution in a direction of achannel is generated on a threshold voltage for a MOS transistor and itis hard to estimate a capacitance value of the variable capacitancebased on a gate voltage.

By the structure according to the third embodiment in which the pocketregions are not provided in the variable capacitance of the inversiontype, accordingly, it is possible to obtain an effect that precision inestimation of the capacitance value of the variable capacitance can beenhanced.

<Fourth Embodiment>

A fourth embodiment provides a method of manufacturing a semiconductordevice which is different from the method of obtaining the structureaccording to the first embodiment. While the steps of forming theextension regions of the MOS transistor and the variable capacitance arecarried out independently in the manufacturing method according to thefirst embodiment, a plurality of extension regions can be formed at thesame time on the same conditions in the fourth embodiment.

FIGS. 12 to 15 are sectional views showing the method of manufacturing asemiconductor device according to the fourth embodiment. With referenceto these drawings, description will be given to a procedure formanufacturing a semiconductor device according to the fourth embodiment.

First of all, the structure shown in FIG. 2 is obtained in the samemanner as in the first embodiment. As shown in FIG. 12, then, a resist55 is formed on a region other than an NMOS formation region A1 and anN-type variable capacitance formation region A3, and an N-type impurityion 67 is thereafter implanted into only the NMOS formation region A1and the N-type variable capacitance formation region A3 by using a gateelectrode 13 and a gate electrode 33 as masks. Consequently, an N⁻extension region 18 and an N⁻ extension region 38 are obtained at thesame time. The N-type impurity ion 67 is implanted at an implantationangle of “0”.

As shown in FIG. 13, subsequently, a resist 56 is formed on a regionother than the NMOS formation region A1 and a P-type impurity ion 68 isthen implanted and diffused into only the NMOS formation region A1 byusing the gate electrode 13 as a mask. Consequently, a P⁻ diffusionregion 19 is formed. The P-type impurity ion 68 is obliquely implantedat a higher implantation energy than that of the N-type impurity ion 67.

As shown in FIG. 14, then, a resist 57 is formed on a region other thana PMOS formation region A2 and a P-type variable capacitance formationregion A4, and a P-type impurity ion 69 is thereafter implanted intoonly the PMOS formation region A2 and the P-type variable capacitanceformation region A4 by using a gate electrode 23 and a gate electrode 43as masks. Consequently, a P⁻ extension region 28 and a P⁻ extensionregion 48 are obtained at the same time. The P-type impurity ion 69 isimplanted at an implantation angle of “0”.

As shown in FIG. 15, subsequently, a resist 58 is formed on a regionother than the PMOS formation region A2 and an N-type impurity ion 70 isthen implanted and diffused into only the PMOS formation region A2 byusing the gate electrode 23 as a mask. Consequently, an N⁻ diffusionregion 29 is formed. The N-type impurity ion 70 is obliquely implantedat a higher implantation energy than that of the P-type impurity ion 69.

Subsequently, it is possible to obtain the structure shown in FIG. 1 byusing the existing method of forming a MOS transistor and a variablecapacitance.

In the method of manufacturing a semiconductor device according to thefourth embodiment, thus, the extension region is simultaneously formedin the MOS transistor and the variable capacitance which have the sameconductivity type. As compared with the method of manufacturing asemiconductor device according to the first embodiment, therefore, twoion implanting steps can be eliminated.

<Fifth Embodiment>

FIG. 16 is a sectional view showing a structure of a semiconductordevice having a MOS transistor and a variable capacitance according to afifth embodiment of the present invention.

As shown in FIG. 16, variable capacitances C5 and C6 having no extensionregion are provided in place of the variable capacitances C1 and C2having the extension regions. More specifically, as compared with thestructure according to the first embodiment shown in FIG. 1, the N⁺extraction electrode regions 34 having the extension portion 34 e isreplaced with an N⁺ extraction electrode regions 35 having no extensionportion and the P⁺ extraction electrode regions 44 having the extensionportion 44 e is replaced with a P⁺ extraction electrode regions 45having no extension portion. Other structures are the same as those ofthe first embodiment shown in FIG. 1.

A method of manufacturing a semiconductor device according to the fifthembodiment is different from the method of manufacturing a semiconductordevice according to the first embodiment in that the steps ofmanufacturing the N⁻ extension region 38 and the P⁻ extension region 48shown in FIGS. 4 and 6 are omitted.

In the method of manufacturing a semiconductor device according to thefifth embodiment, thus, the step of forming the extension region of avariable capacitance is omitted. Consequently, two steps, that is, thestep of forming a resist and the step of implanting an ion can beeliminated as compared with the method of manufacturing a semiconductordevice according to the first embodiment.

Although the variable capacitance cannot produce the effect by theprovision of the extension portion in the semiconductor device accordingto the fifth embodiment, the effect of providing no pocket region can beobtained in the same manner as that in each of the first to fourthembodiments.

<Sixth Embodiment>

FIG. 17 is a sectional view showing a structure of a semiconductordevice having a MOS transistor and a variable capacitance according to asixth embodiment of the present invention.

As shown in FIG. 17, variable capacitances C7 and C8 having extensionregions in higher concentrations than those of the variable capacitancesC1 and C2 are provided in place of the variable capacitances C1 and C2.

More specifically, as compared with the structure according to the firstembodiment, the N⁻ extension region 38 and the P⁻ extension region 48shown in FIGS. 4 and 6 are formed to have impurity concentrations whichare approximately twice to 100 times as high as those of the N⁻extension region 18 and the P⁻ extension region 28 in the MOStransistor.

Although gate lengths of MOS transistors Q1 and Q2 are almost equal tothose of the variable capacitances C7 and C8 in FIG. 17, the gate lengthof the variable capacitance is actually set to be greater than that ofthe MOS transistor in many cases.

Accordingly, the influence of a short channel effect in the variablecapacitance is smaller than that in the MOS transistor. Consequently, abad effect is lessened with an increase in the impurity concentration ofthe extension region. To the contrary, it is possible to obtain agreater advantage that a series resistance component of the variablecapacitance can be reduced with the increase in the impurityconcentration of the extension region.

While the example in which the extension region is formed in a highconcentration has been described in the present embodiment, the sameeffects can be obtained even if an implantation energy of an impurityion is increased to be approximately 1.2 to 30 times as high as that ofthe MOS transistor and a depth of the extension region is increased tobe approximately 1.2 to 30 times as great as that of the MOS transistor.

<Seventh Embodiment>

In general, if a device is subjected to scaling, a power supply voltageis also subjected to the scaling and is thereby reduced. Consequently,it is necessary to provide an interface with another chip (device) to beoperated at a high voltage.

At this time, it is necessary to fabricate a 3.3 V or 5.0 V compatibleMOS transistor for a high voltage (hereinafter referred to as a“transistor for a high voltage”), for example, in addition to a highperformance MOS transistor (hereinafter referred to as a “highperformance transistor”) which is subjected to the scaling in thedevice.

By a comparison of the transistor for a high voltage with the highperformance transistor, a gate length is greater and a gate oxide filmhas a greater thickness, and furthermore, an extension region is formedon different conditions and pocket regions are not formed in many cases.The extension region is formed on the different conditions in order toincrease a hot carrier tolerance such that an S/D breakdown phenomenonsuch as punch-through is not presented at a high voltage. The pocketregions do not need to be formed because the gate length is great.

FIG. 18 is a diagram illustrating a layout structure of a semiconductordevice having a MOS transistor and a variable capacitance according to aseventh embodiment of the present invention. As shown in FIG. 18, thesemiconductor device is constituted by a high performance transistorformation region E1, a high voltage transistor formation region E2 and avariable capacitance formation region E3 according to the seventhembodiment, and a high performance transistor, a transistor for a highvoltage and a variable capacitance are provided in the formation regionsE1 to E3, respectively.

FIG. 19 is a sectional view showing structures of the high voltagetransistor formation region E2 and the variable capacitance formationregion E3 in the semiconductor device according to the seventhembodiment.

In an NMOS formation region A5 of the high voltage transistor formationregion E2, a gate oxide film 72 is selectively formed on a surface of aP well region 71 to be a body region and an N⁺-type gate electrode 73 isformed on the gate oxide film 72. N⁺ source-drain regions 74 and 74 areformed to interpose therebetween a channel region provided under thegate electrode 73 and tip regions opposed to each other between the N⁺source-drain regions 74 and 74 are extension portions 74 e,respectively.

Moreover, side walls 76 and 76 are formed on both side surfaces of thegate electrode 73, respectively. Furthermore, a silicide region 74 s anda silicide region 73 s are formed in a surface of the N⁺ source-drainregions 74 and an upper layer portion of the gate electrode 73,respectively.

Thus, an NMOS transistor Q3 for a high voltage is formed by the gateoxide film 72, the gate electrode 73, the N⁺ source-drain regions 74 andthe side wall 76.

In a PMOS formation region A6 of the high voltage transistor formationregion E2, a gate oxide film 82 is selectively formed on a surface of anN well region 81 and a P⁺-type gate electrode 83 is formed on the gateoxide film 82. P⁺ source-drain regions 84 and 84 are formed to interposetherebetween a channel region provided under the gate electrode 83 andtip regions opposed to each other between the P⁺ source-drain regions 84and 84 are extension portions 84 e, respectively.

Moreover, side walls 86 and 86 are formed on both side surfaces of thegate electrode 83, respectively. Furthermore, a silicide region 84 s anda silicide region 83 s are formed in a surface of the P⁺ source-drainregions 84 and an upper layer portion of the gate electrode 83,respectively.

Thus, a PMOS transistor Q4 for a high voltage is formed by the gateoxide film 82, the gate electrode 83, the P⁺ source-drain regions 84 andthe side wall 86.

The high performance transistor formed in the high performancetransistor formation region E1 is provided to have the same structure asthat of each of the NMOS transistor Q1 and the PMOS transistor Q2according to the first embodiment shown in FIG. 1, for example, which isnot shown in FIG. 19.

The NMOS transistor Q3 for a high voltage and the PMOS transistor Q4 fora high voltage are different from the NMOS transistor Q1 and the PMOStransistor Q2 for a high performance in that gate lengths are greater,the gate oxide films have greater thicknesses, the extension regions areset on different conditions and the pocket regions are not formed.

On the other hand, an N-type variable capacitance C1 and a P-typevariable capacitance C2 are formed in an N-type variable capacitanceformation region A3 and a P-type variable capacitance formation regionA4 in the variable capacitance formation region E3, respectively.

The N-type variable capacitance C1 and the P-type variable capacitanceC2 have the same basic structures as those of the N-type variablecapacitance C1 and the P-type variable capacitance C2 according to thefirst embodiment shown in FIG. 1.

In each of the N-type variable capacitance C1 and the P-type variablecapacitance C2, an extension region is formed to have a higher impurityconcentration than that of the extension region of each of the NMOStransistor Q1 and the PMOS transistor Q2 and an equal gate length on thesame conditions (at least the impurity concentration is almost equal) ascompared with the NMOS transistor Q3 for a high voltage and the PMOStransistor Q4 for a high voltage. In the N-type variable capacitance C1and the P-type variable capacitance C2, moreover, the gate oxide filmshave equal thicknesses as compared with the NMOS transistor Q1 and thePMOS transistor Q2.

In the semiconductor device having such a structure according to theseventh embodiment, the extension regions in the NMOS transistor Q3 fora high voltage and the N-type variable capacitance C1 can be formed atthe same step and the extension regions of the PMOS transistor Q4 for ahigh voltage and the P-type variable capacitance C2 can be formed at thesame step. Therefore, it is possible to obtain a semiconductor devicehaving a variable capacitance with a series resistance component reducedwhile minimizing the number of manufacturing steps.

In some cases, moreover, the extension region of the transistor for ahigh voltage is to be formed as an LDD region to have an impurityconcentration which is almost equal to that of the extension region ofthe high performance transistor. In these cases, an implantation energyis increased and the extension region is formed deeply.

Accordingly, it is possible to obtain the same effects as those in thesemiconductor device according to the sixth embodiment by forming theextension region of the variable capacitance comparatively deeply on thesame conditions as those of the extension region of the transistor for ahigh voltage.

<Eighth Embodiment>

An eighth embodiment provides a semiconductor device having such astructure as to comprise a MOS transistor and a variable capacitance inwhich channel regions have different impurity concentrations,respectively. In the semiconductor device according to the eighthembodiment, the impurity concentrations of the channel regions in theMOS transistor and the variable capacitance are set to be different fromeach other so that a degree of freedom of a design in the device can beenhanced, for example, a threshold voltage can be set separately.

FIG. 20 is a sectional view showing a structure of the semiconductordevice having the MOS transistor and the variable capacitance accordingto the eighth embodiment of the present invention. In FIG. 20, a PMOStransistor Q2 to be formed in a PMOS formation region A2. is the same asthe PMOS transistor Q2 according to the first embodiment shown in FIG.1.

On the other hand, an N-type variable capacitance C9 to be formed in anN-type variable capacitance formation region A3 has P⁻ pocket regions 37in the vicinity of an extension portion 34 e and an N well region 31provided between N⁺ extraction electrode regions 34 and 34 acts as ahigh concentration channel region 31 c. Other structures are the same asthe structure of the N-type variable capacitance C1 according to thefirst embodiment shown in FIG. 1.

The N-type variable capacitance C9 includes the high concentrationchannel region 31 c having a higher N-type impurity concentration thanthat of other regions of the N well region 31. The high concentrationchannel region 31 c can cancel the P⁻ pocket regions 37 to sufficientlycompensate for a reduction in a series resistance component. Therefore,a Q-value of the variable capacitance can be fully increased.

In the structure shown in FIG. 20, thus, when a channel concentration isto be changed between the PMOS transistor Q2 and the N-type variablecapacitance C9, the high concentration channel region 31 c is providedin the N-type variable capacitance C9, thereby increasing the Q-value ofthe variable capacitance. More specifically, the structure shown in FIG.20 is a desirable example in which impurity concentrations in therespective channel regions of the MOS transistor and the variablecapacitance are set to be different from each other, thereby enhancing adegree of freedom of a design.

After the N well region 31 is formed, an N-type impurity is furtherimplanted into an upper layer portion of the N well region 31 to obtainthe high concentration channel region 31 c. More specifically, a step offorming the high concentration channel region 31 c is requiredseparately.

While only the PMOS transistor and the N-type variable capacitance areshown in FIG. 20, it is a matter of course that an NMOS transistor and aP-type variable capacitance can also be formed to have the samestructures.

<Ninth Embodiment>

FIG. 21 is a sectional view showing a part of a method of manufacturinga semiconductor device having a MOS transistor and a variablecapacitance according to a ninth embodiment of the present invention.

In the method of manufacturing a semiconductor device according to theninth embodiment, pocket regions are formed in both the MOS transistorand the variable capacitance.

It is assumed that source-drain regions are formed on the MOS transistorside and extraction electrode regions 34 are formed on the variablecapacitance side by first ion implantation and diffusion through aforming step based on an existing method.

For the first impurity implantation and diffusion, a heat treatment (forexample, RTA (Rapid Thermal Anneal) at 900 to 1100° C. for 10 to 120seconds in an N₂ atmosphere) is carried out after implantation of anN-type impurity. For the heat treatment, a crystal defect formed by theimplantation of the N-type impurity is recovered.

Subsequent processings are peculiar to the manufacturing methodaccording to the ninth embodiment. In the ninth embodiment, furthermore,second impurity implantation and diffusion is not carried out over theMOS transistor but only the variable capacitance as shown in FIG. 21. Inan example of FIG. 21, for the second impurity implantation, an N-typeimpurity ion 91 is implanted by using a gate electrode 33 as a mask tocarry out a heat treatment. Consequently, N⁺ extraction electroderegions 34 h are formed so that an N-type variable capacitance C15 isfinally obtained.

Examples of the second impurity implantation and diffusion includeannealing to be carried out at a comparatively low temperature of 500 to800° C. for approximately 10 to 120 minutes after the implantation ofthe N-type impurity.

The second impurity implantation and diffusion is carried out over onlythe variable capacitance and the heat treatment is performed at thecomparatively low temperature during the diffusion as described above.Therefore, there is presented TED (Transient Enhanced Diffusion) to be aphenomenon in which a crystal defect formed by the second impurity ionimplantation is introduced into a well region of the variablecapacitance and a defect portion and an impurity are coupled to eachother and are greatly diffused.

By the TED phenomenon, impurities in P⁻ pocket regions 37 and the N⁺extraction electrode regions 34 h which are formed in the N-typevariable capacitance C15 are diffused again. As a result, the presenceof the P⁻ pocket regions 37 do not have such an influence that a seriesresistance is reduced. Consequently, it is possible to obtain the N-typevariable capacitance C15 having a great Q-value.

While FIG. 21 shows the N-type variable capacitance C15, it is a matterof course that the present invention can also be applied to a P-typevariable capacitance.

The second implantation may include a process of implanting an impurityion of the N type in an oblique direction.

In this case, the deterioration in the resistance component by thepocket regions 37 can be effectively suppressed by the ion implantationin the oblique direction.

<Tenth Embodiment>

FIGS. 22 to 26 are sectional views showing a method of manufacturing anN-type variable capacitance in a semiconductor device having a MOStransistor and a variable capacitance according to a tenth embodiment ofthe present invention. With reference to these drawings, descriptionwill be given to a procedure for manufacturing the N-type variablecapacitance according to the tenth embodiment.

As shown in FIG. 22, first of all, a lamination structure having a gateoxide film 32, a gate electrode 33 and an oxide film 59 for a mask isselectively provided on a surface of an N well region 31. By using thelamination structure as a mask, N- and P-type impurities are introducedto form an N⁻ extension region 38 and a P⁻ diffusion region 39,respectively. For a material of the gate electrode 33 to be formed,polysilicon is used.

As shown in FIG. 23, next, isotropic polysilicon etching is carried outover the gate electrode 33 to partially remove a peripheral region in adirection of a gate length of the gate electrode 33. Thus, a gateelectrode 33 n having a small gate length is obtained.

As shown in FIG. 24, then, wet etching for an oxide film is carried outover the oxide film 59 for a mask and the gate oxide film 32.Consequently, an oxide film 59 n for a mask and a gate oxide film 32 nare obtained by reducing the oxide film 59 for a mask and the gate oxidefilm 32.

As shown in FIG. 25, then, a side wall 36 is formed on a side surface ofthe gate electrode 33 n.

As shown in FIG. 26, thereafter, an N-type impurity ion 75 is implantedand diffused by using the gate electrode 33 n and the side wall 36 asmasks. Consequently, N⁺ extraction electrode regions 34 d are obtained.The N⁺ extraction electrode regions 34 d are formed in a regionincluding a whole P⁻ diffusion region 39 and an N-type impurityconcentration is higher than a P-type impurity concentration of the P⁻diffusion region 39. Therefore, the influence of the P⁻ diffusion region39 can be cancelled completely. More specifically, pocket regions arenot present in the finished variable capacitance.

In the tenth embodiment, thus, the side wall is formed and theextraction electrode regions are provided after the gate length of thegate electrode is reduced. Consequently, the finished device can havesuch a structure that the pocket regions are not present. Therefore, itis possible to obtain an N-type variable capacitance having a greatQ-value even if pocket regions forming step is included.

While the method of manufacturing the N-type variable capacitance hasbeen described in the tenth embodiment, it is a matter of course that aP-type variable capacitance can be manufactured in the same manner.

<Eleventh Embodiment>

FIG. 27 is a sectional view showing a structure of a semiconductordevice having a MOS transistor and a variable capacitance according toan eleventh embodiment of the present invention. In FIG. 27, an NMOStransistor Q1 to be formed in an NMOS formation region A1 is the same asthe NMOS transistor Q1 according to the first embodiment shown in FIG.1.

On the other hand, an N-type variable capacitance C1 w to be formed inan N-type variable capacitance formation region A3 is different in thata thickness of a gate oxide film 32 w is greater than that of a gateoxide film 12. Other structures are the same as those of the N-typevariable capacitance C1 according to the first embodiment shown in FIG.1.

An oscillation frequency f of an LC type VCO is determined by thefollowing equation (2). Therefore, it is desirable that a capacitancecomponent of a variable capacitance should be reduced in order tofabricate an oscillator for oscillation at a high frequency.$\begin{matrix}{{\left\lbrack {{Equation}\quad 2} \right\rbrack\quad\quad{f = \frac{1}{2\pi\sqrt{L\quad C}}}}\quad} & (2)\end{matrix}$

However, there is a problem in that a series parasitic resistance isincreased if a variable capacitance is manufactured in a small pattern.

As shown in FIG. 27, when the gate oxide film 32 w is formed to have agreater thickness than that of the gate oxide film 12, the capacitancecomponent of the variable capacitance can be reduced without changing apattern size, that is, increasing a parasitic resistance component.Furthermore, a Q-value can also be enhanced by a reduction in thecapacitance component in accordance with the equation (1).

Moreover, in the case in which a transistor for a high voltage isprovided in addition to a high performance transistor as in thesemiconductor device according to the seventh embodiment, the gate oxidefilm 32 w is formed during formation of a gate oxide film of thetransistor for a high voltage which has a greater thickness than that ofa gate oxide film in the high performance transistor. Consequently, itis possible to obtain the gate oxide film 32 w having a smallerthickness than that of the high performance transistor withoutincreasing the number of manufacturing steps.

While only the NMOS transistor and the N-type variable capacitance areshown in FIG. 27, it is a matter of course that a PMOS transistor and aP-type variable capacitance can also be formed to have the samestructures.

<Twelfth Embodiment>

(First Mode)

FIG. 28 is a sectional view showing a structure of a first mode of asemiconductor device having a MOS transistor and a variable capacitanceaccording to a twelfth embodiment of the present invention. In FIG. 28,an NMOS transistor Q1 to be formed in an NMOS formation region A1 is thesame as the NMOS transistor Q1 according to the first embodiment shownin FIG. 1.

On the other hand, an N-type variable capacitance C1L to be formed in anN-type variable capacitance formation region A3 is different in that amaterial of a gate oxide film 32L has a lower dielectric constant thanthat of a material of a gate oxide film 12. Other structures are thesame as those of the N-type variable capacitance C1 according to thefirst embodiment shown in FIG. 1.

In order to obtain the gate oxide film 32L, for example, it is proposedthat F (fluorine) is implanted into only a gate oxide film 32 of theN-type variable capacitance C1L.

In the first mode of the twelfth embodiment, thus, a capacitancecomponent of the variable capacitance can be reduced without increasinga parasitic resistance component. Therefore, the same effects as thosein the eleventh embodiment can be obtained.

While only the NMOS transistor and the N-type variable capacitance areshown in FIG. 28, it is a matter of course that a PMOS transistor and aP-type variable capacitance can be formed to have the same structures.

(Second Mode)

FIG. 29 is a sectional view showing a structure of a second mode of thesemiconductor device having a MOS transistor and a variable capacitanceaccording to a twelfth embodiment of the present invention. In FIG. 29,an N-type variable capacitance C1 w to be formed in an N-type variablecapacitance formation region A3 is different in that a thickness of agate oxide film 32 w is greater than that of a gate oxide film 12. Otherstructures are the same as those of the N-type variable capacitance C1according to the first embodiment shown in FIG. 1.

An NMOS transistor Q1 to be formed in an NMOS formation region A1 isdifferent in that a gate insulating film 12H is formed by using a High—kmaterial having a higher dielectric constant than that of a siliconoxide film and the gate insulating film 12H is formed to have an almostequal thickness to that of the gate oxide film 32 w. Other structuresare the same as those of the NMOS transistor Q1 according to the firstembodiment shown in FIG. 1.

Examples of the High—k material include Si₃N₄, Ta₂O₅, Al₂O₃, HfO₂, ZrO₂and the like.

In the first mode of the twelfth embodiment, thus, it is possible toreduce a capacitance component of the variable capacitance withoutincreasing a parasitic resistance component. Therefore, the same effectsas those in the eleventh embodiment can be obtained.

The thickness of the gate insulating film 12H is almost equal to that ofthe gate oxide film 32 w. Therefore, the gate insulating film 12H andthe gate oxide film 32 w can be manufactured at the same step.Consequently, it is possible to obtain the gate oxide film 322 having asmaller thickness than that of a high performance transistor withoutincreasing the number of manufacturing steps. In this case, since thegate insulating film 12H is formed of the High-k material, an electricalcharacteristic of an NMOS transistor Q1H is not adversely affected.

While only the NMOS transistor and the N-type variable capacitance areshown in FIG. 29, it is a matter of course that a PMOS transistor and aP-type variable capacitance can be formed to have the same structures.

<Thirteenth Embodiment>

FIG. 30 is a sectional view showing a structure of a semiconductordevice having a MOS transistor and a variable capacitance according to athirteenth embodiment of the present invention. In FIG. 30, an N-typevariable capacitance C1 p to be formed in an N-type variable capacitanceformation region A3 is different in that a porous silicon layer 8 isformed in an upper layer portion of an N well region 31. Otherstructures are the same as those of the N-type variable capacitance C1according to the first embodiment shown in FIG. 1.

By the provision of the porous silicon layer 8, an effective dielectricconstant of silicon is decreased so that a capacitance component of theN-type variable capacitance C1 p can be reduced. When a vacancy iscontinuously formed so that a rate (vacancy ratio) of the vacancyoccupying the upper layer portion of the N well region 31 is too high, aresistance of the N well region 31 is increased. Therefore, it isdesirable that the vacancy ratio should be 50% or less.

Since a capacitance component of the variable capacitance can be thusreduced without greatly increasing a parasitic resistance component inthe thirteenth embodiment, the same effects as those in the eleventhembodiment can be obtained.

While only the N-type variable capacitance is shown in FIG. 30, it is amatter of course that a P-type variable capacitance can also be formedto have the same structure.

Formation of Porous Silicon Layer

FIGS. 31 to 35 are sectional views showing a method of forming a poroussilicon layer which has been disclosed in Japanese Patent ApplicationLaid-Open No. 2000-307112, for example. With reference to thesedrawings, a procedure for forming the porous silicon layer will bedescribed below.

As shown in FIG. 31, first of all, a porous silicon layer 7 is formed inan upper surface of an N-type silicon substrate 6 by anode formation.More specifically, the silicon substrate 6 is immersed in an HF solution152 in a formation layer 151 and a current is caused to flow to thesilicon substrate 6 by setting an upper platinum electrode 153 to be acathode and a lower platinum electrode 154 to be an anode. Forconditions, a formation time of 30 seconds and a formation currentdensity of 10 mA/cm² are set. As shown in FIG. 32, consequently, theupper surface of the silicon substrate 6 is made porous and the poroussilicon layer 7 having a thickness of approximately 0.2 μm is formed inthe upper surface of the silicon substrate 6.

FIG. 33 is a sectional view specifically showing a shape of the poroussilicon layer 7. The porous silicon layer 7 has a complicated shape asshown in FIG. 33 (more specifically, see Document 2, pp 470, FIG. 4 orDocument 3, pp 379, FIG. 2 which will be described below). In thisspecification, the shape of the porous silicon layer 7 is simplified fordescription as shown in FIG. 32. A thickness of the porous silicon layer7 can be controlled based on a formation time and a formation currentdensity, and furthermore, the vacancy ratio of the porous silicon layer7 (a density corresponding to a ratio of a silicon portion 7 a to avacancy portion 7 b) can be controlled by a concentration of the HFsolution 152 (see SOI structure forming technique, pp 181 to 185,written by Seijiro Furukawa, 1987, Sangyo Tosho: (Document 1)).

In order to maintain stability of the porous structure of the poroussilicon layer 7 for a heat treatment, next, preoxidation is carried outat a low temperature of approximately 400° C. In order to reduce thequantity of crystal defects of an expitaxial layer 9 to be formed at asubsequent step, then, the heat treatment is carried out at atemperature of 1000° C. or more for a few seconds in a hydrogenatmosphere. Consequently, a mobility of surface atoms is dramaticallyincreased by minimization of a surface energy of the porous siliconlayer 7 and a surface hole (not shown) generated in an upper surface ofthe porous silicon layer 7 due to natural oxidation of a surface isreduced and removed. As a result, as shown in FIG. 34, a porous siliconlayer 8 is formed by sufficiently smoothening the upper surface of theporous silicon layer 7.

An upper surface of the porous silicon layer 8 maintains a singlecrystal structure of the silicon substrate 6 and has the same crystalorientation as that of the silicon substrate 6. As shown in FIG. 35, theepitaxial layer 9 having a thickness of approximately 100 nm is formedon the upper surface of the porous silicon layer 8 by an epitaxialgrowth method. See “Science of Silicon, pp 467-475, edited by TadahiroOhmi et al., REALIZE INC.” (Document 2), “IEICE TRANS. ELECTRON., VOL.E80-C, NO. 3, MARCH 1997, K.SAKAGUCHI et al., pp 378-387” (Document 3),and “Extended Abstracts of the 1998 International Conference on SolidState Devices and Materials, Hiroshima, 1998, pp 302-303” (Document 4)for the epitaxial growth of silicon on the porous silicon layer.

In the thirteenth embodiment, the porous silicon layer 8 is selectivelyformed in the N-type variable capacitance formation region A3 and aP-type variable capacitance formation region A4. Thus, in the case inwhich porous silicon is to be partially formed, surfaces of an NMOSformation region A1 and a PMOS formation region A2 are covered with aresist mask during the anode formation shown in FIG. 31 such that theporous silicon layer 7 is not formed.

<Fourteenth Embodiment>

A fourteenth embodiment provides a parallel connection type variablecapacitance in which an effective Q-value is enhanced by connecting ajunction capacitance type variable capacitance (hereinafter referred toas a “junction type variable capacitance”) having a great Q-value to aninsulated gate type capacitance in parallel because an enhancement inthe Q-value is limited with only the insulated gate type capacitance.

FIG. 36 is a sectional view showing a structure of a junction typevariable capacitance. As shown in FIG. 36, an N⁺ diffusion region 162 isformed in an upper layer portion of a P⁻ substrate 161 to be a bulksubstrate so that a junction type variable capacitance C21 having a PNjunction surface between the P⁻ substrate 161 and the N⁺ diffusionregion 162 can be obtained.

FIG. 37 is a diagram illustrating an equivalent circuit of the parallelconnection type variable capacitance according to the fourteenthembodiment. As shown in FIG. 37, a junction type variable capacitance CJ(C21) having a great Q-value and an insulated gate type capacitance CMare connected to each other in parallel between a terminal PA and aterminal PB (PB1, PB2). Consequently, an effective Q-value can beincreased. Different electric potentials or an equal electric potentialmay be applied to the terminals PB1 and PB2.

The junction type variable capacitance CJ can variably set a capacitancevalue depending on a voltage to be applied to the terminal PB1. Morespecifically, a PN junction is the sum of a depletion layer capacitanceand a diffusion capacitance, and the diffusion capacitance can bedisregarded in backward biasing and the depletion layer capacitance hasa bias voltage dependency. Therefore, a junction capacitance can be usedas the variable capacitance.

The insulated gate type variable capacitance CM corresponds to any ofthe insulated gate type capacitances according to the first tothirteenth embodiments, for example. In this case, a fixed electricpotential is applied to an insulated gate (an insulated gate receives afixed electric potential) so that the insulated gate type variablecapacitance CM can be utilized as a fixed capacitance having a structureusing an insulated gate structure. More specifically, the circuitstructure shown in FIG. 37 can be regarded as a parallel connection ofthe variable capacitance and the fixed capacitance.

FIG. 38 is a sectional view showing a structure of a junction typevariable capacitance in a bulk substrate having a structure in which anisolation is carried out. As shown in FIG. 38, an isolating oxide film163 (163 a to 163 c) is selectively formed in the upper layer portion ofthe P⁻ substrate 161 so that an isolation structure can be obtained.

The N⁺ diffusion region 162 is formed on the P⁻ substrate 161 betweenthe isolating oxide films 163 a and 163 b and a P⁺ diffusion region 164is formed on the P⁻ substrate 161 between the isolating oxide films 163b and 163 c. More specifically, a junction type variable capacitance C22having a PN junction surface of the P⁻ substrate 161 and the N⁺diffusion region 162 is obtained.

In the junction type variable capacitance, the N⁺ diffusion region 162and the P⁺ diffusion region 164 are isolated from each other through theisolating oxide film 163. Therefore, it is possible to obtain thejunction type variable capacitance C22 having a sufficient breakdownvoltage.

FIG. 39 is a plan view showing a planar structure of the junction typevariable capacitance. An A—A section in FIG. 39 corresponds to FIG. 38.

As shown in FIG. 39, a formation width W between the N⁺ diffusion region162 and the P⁺ diffusion region 164 is sufficiently increased so that ad.c. resistance component between the terminals PA and PB1 can be fullyreduced.

<Fifteenth Embodiment>

FIG. 40 is a sectional view showing a structure of a junction typevariable capacitance according to a fifteenth embodiment of the presentinvention. As shown in FIG. 40, the junction type variable capacitanceaccording to the fifteenth embodiment is formed on an SOI substratecomprising a support substrate 165, a buried oxide film 166 and an SOIlayer 171.

As shown in FIG. 40, the buried oxide film 166 is formed on the supportsubstrate 165 and the SOI layer 171 is provided on the buried oxide film166. Then, an isolating oxide film 167 (167 a to 167 c) is selectivelyformed in an upper layer portion of the SOI layer 171 with a part of theSOI layer 171 remaining as a P⁻ well region 169. Thus, an isolation(partial isolation) structure can be obtained.

Subsequently, an N⁺ diffusion region 168 is formed in the SOI layer 171between the isolating oxide films 167 a and 167 b and a P⁺ diffusionregion 170 is formed in the SOI layer 171 between the isolating oxidefilms 167 b and 167 c. Accordingly, a junction type variable capacitanceC23 having a PN junction surface of the P⁻ well region 169 and the N⁺diffusion region 168 can be obtained.

By the presence of the buried oxide film 166, the PN junction surface isprovided on only side surfaces of the P⁻ well region 169 and the N⁺diffusion region 168. For this reason, it is necessary to increase aformation region on the side surfaces of the N⁺ diffusion region 168 andthe P⁻ well region 169, thereby obtaining a necessary junctioncapacitance.

However, if the formation area on the side surface is made larger toincrease the formation areas of the P⁻ well region 169 and the P⁺diffusion region 170, a parasitic capacitance is generated between thesupport substrate 165 and the buried oxide film 166. If an AC currentflows to the support substrate 165 through the parasitic capacitance, asignal loss might be caused by a resistance component of the supportsubstrate 165 and an influence thereof is small in the SOI structure.

By increasing a formation width of the PN junction surface in the N⁺diffusion region 168 and the P⁻ well region 169 in the same manner as aformation width W of the N⁻ diffusion region 162 or the like in FIG. 39,moreover, the junction type variable capacitance can be increased and ad.c. resistance component can be sufficiently reduced. As a result, aQ-value can be enhanced.

The junction type variable capacitance according to the fifteenthembodiment may be used independently and may be connected to aninsulated gate type capacitance in parallel as in the fourteenthembodiment.

<Sixteenth Embodiment>

(First Mode)

FIG. 41 is a sectional view showing a structure of a first mode of ajunction type variable capacitance according to a sixteenth embodimentof the present invention. As shown in FIG. 41, an N⁺ diffusion region174 and a P⁺ diffusion region 175 are selectively formed on a P⁻substrate 161 to be a bulk substrate and a gate electrode 173 is formedon the P⁻ substrate 161 between the N⁺ diffusion region 174 and the P⁺diffusion region 175 through a gate oxide film 172.

Accordingly, a junction type variable capacitance C24 having a PNjunction surface of the N⁺ diffusion region 174 and the P⁻ substrate 161is obtained.

In the first mode, in the ease in which the N⁺ diffusion region 174 andthe P⁺ diffusion region 175 are to be provided after the formation ofthe gate electrode 173, a surface (body region) of the P⁻ substrate 161provided under the gate electrode 173 is masked by the gate electrode173. Consequently, it is possible to reduce an impurity concentration inthe body region comparatively easily.

As a result, a PN junction of regions having high concentrations can beavoided so that the junction type variable capacitance C24 having asufficient breakdown voltage can be obtained.

(Second Mode)

FIG. 42 is a sectional view showing a structure of a second mode of thejunction type variable capacitance according to the sixteenth embodimentof the present invention. As shown in FIG. 42, an N⁺ diffusion region176 and a P⁺ diffusion region 178 are selectively formed in an SOI layer171 of an SOI substrate comprising a support substrate 165, a buriedoxide film 166 and the SOI layer 171, and a gate electrode 183 is formedon a P⁻ well region 177 to be a region of the SOI layer 171 between theN⁺ diffusion region 176 and the P⁺ diffusion region 178 through a gateoxide film 182.

Accordingly, a junction type variable capacitance C25 having a PNjunction surface of the N⁺ diffusion region 176 and the P⁻ well region177 is obtained.

Also in the second mode, in the same manner as in the first mode, the P⁻well region 177 (body region) provided under the gate electrode 183 ismasked by the gate electrode 183. Consequently, it is possible to reducean impurity concentration of the P⁻ well region 177 comparativelyeasily.

As a result, a PN junction of regions having high concentrations can beavoided so that the junction type variable capacitance C25 having asufficient breakdown voltage can be obtained.

(Third Mode)

FIG. 43 is a sectional view showing a structure of a junction typevariable capacitance C26 according to a third mode of the sixteenthembodiment of the present invention. A shown in FIG. 43, an N⁺ diffusionregion 174 and a P⁺ diffusion region 175 are selectively formed in anupper layer portion of a P⁻ substrate 161, and silicide regions 180 and181 are formed in upper layer portions of the N⁺ diffusion region 174and the P⁺ diffusion region 175, respectively.

Then, a silicide protection 187 is provided on the P⁻ substrate 161between the silicide regions 180 and 181.

(Fourth Mode)

FIG. 44 is a sectional view showing a structure of a junction typevariable capacitance C27 according to a fourth mode of the sixteenthembodiment of the present invention. As shown in FIG. 44, silicideregions 184 and 185 are formed in upper layer portions of an N⁺diffusion region 176 and a P⁺ diffusion region 178, respectively.

Then, a silicide protection 188 is provided on an SOI layer 171 betweenthe silicide regions 184 and 185. Other structures are the same as thosein the second mode shown in FIG. 42.

Thus, the first and second modes of the sixteenth embodiment provide thesame structure as that of the MOS transistor excluding conductivitytypes of the P⁺ diffusion regions 175 and 178. Therefore, it is possibleto carry out manufacture by using most of the process for manufacturingthe MOS transistor.

In the third and fourth modes, moreover, the silicide protections 187and 188 are provided to form the silicide region. Consequently, aresistance can be reduced.

Also in the first and second modes, a side wall can be formed on a sidesurface of the gate electrode 173 (183) and the silicide region can beformed in the upper layer portions of the N⁺ diffusion region 174 (176)and the P⁺ diffusion region 175 (178) by using the gate electrode andthe side wall as the silicide protections. Also in the sixteenthembodiment shown in FIG. 40, moreover, it is possible to form thesilicide region on surfaces of the N⁺ diffusion region 168 and the P⁺diffusion region 170 by an existing method.

It is desirable that the junction type variable capacitances accordingto the first to fourth modes should be connected to an insulated gatetype capacitance in parallel as in the fourteenth embodiment. Moreover,the second and fourth modes may be used independently.

While the PN junction of N⁺/P⁻ formed on the P⁻ substrate (well region)has been described in the first to fourth modes, a PN junction of P⁺/N⁻formed on an N⁻ substrate (well region) may be used or a junction typevariable capacitance may be formed on the N⁻ substrate.

<Seventeenth Embodiment>

(Structure)

FIG. 45 is a sectional view showing a structure of an MIM (MetalInsulator Metal) type capacitance according to a seventeenth embodimentof the present invention.

As shown in FIG. 45, a TiN electrode 193 is formed on an A1 electrode191 through a p (plasma)-SiN film 192. The TiN electrode 193 iselectrically connected to an aluminum wiring 197 via a through hole 195provided in an interlayer insulating film 194. Moreover, the A1electrode 191 is electrically connected to an aluminum wiring 198 via athrough hole 196 which is provided independently of the through hole 195and penetrates through the interlayer insulating film 194.

In the seventeenth embodiment, thus, an MIM type capacitance C28 isformed by the A1 electrode 191, the p-SiN film 192 and the TiN electrode193. It is desirable that the MIM type capacitance should be connectedto an insulated gate type capacitance in parallel as in the junctiontype variable capacitance according to the fourteenth embodiment.

While the A1 electrode 191 and the TiN electrode 193 are formed aselectrodes in FIG. 45, a resistance component can be further reduced byusing copper (Cu). Moreover, if a ferroelectric film is used in place ofthe p-SiN film 192, a capacitance component can be increased.Alternatively, a formation area can be reduced with the same capacitancecomponent.

(Application Example)

FIG. 46 is a circuit diagram showing an example of a VCO. As shown inFIG. 46, a coil L1 and an NMOS transistor Q21 are connected to a coil L2and an NMOS transistor Q22 in parallel between nodes N1 and N2,respectively. The node N1 is connected to a terminal P1 and the node N2is grounded through an NMOS transistor Q23. A low voltage V23 is appliedto a gate of the NMOS transistor Q23.

A gate of the NMOS transistor Q21 is connected to a node N4, a gate ofthe NMOS transistor Q22 is connected to a node N3, one of ends of eachof a fixed capacitance Cf1 and a variable capacitance Cv1 is connectedto the node N3 in parallel, a constant voltage Vf1 is applied to aterminal P2F of the fixed capacitance Cf1, and a control voltage VC isapplied to a terminal P2 of the variable capacitance Cv1. On the otherhand, one of ends of each of a fixed capacitance Cf2 and a variablecapacitance Cv2 is connected to the node N4 in parallel, a constantvoltage Vf2 is applied to a terminal P3F of the fixed capacitance Cf2,and the control voltage VC is applied to a terminal P3 of the variablecapacitance Cv2.

The VCO having such a structure is operated as an LC type oscillator forchanging capacitance values of the variable capacitances Cv1 and Cv2 bythe control voltage VC.

For the variable capacitance Cv1, the junction capacitance, theinsulated gate type capacitance or the like can be proposed. For thefixed capacitance Cf1, the insulated gate type capacitance having afixed electric potential of a gate electrode, the MIM type capacitance,a PIP type capacitance which will be described below or the like can beproposed.

The control voltage VC is applied to the terminals P2 and P3, and thevariable capacitances Cv of the variable capacitance Cv1 and thevariable capacitance Cv2 are changed according to the control voltageVC. On the other hand, fixed capacitances Cf of the fixed capacitanceCf1 and the fixed capacitance Cf2 are constant.

More specifically, C in the equation (2) is determined by the sum of thevariable capacitance Cv and the fixed capacitance Cf. Therefore, theequation (2) can be expressed in an equation (3). Accordingly, it ispreferable that the variable capacitance Cv should be set by the controlvoltage VC such that (Cv+Cf) has a desirable value. $\begin{matrix}{{\left\lbrack {{Equation}\quad 3} \right\rbrack\quad\quad{f = \frac{1}{2\pi\sqrt{L\quad\left( {{C\quad v} + {Cf}} \right)}}}}\quad} & (3)\end{matrix}$

Description will be given to control for reducing df/dVC to be an amountof a change in an oscillation frequency f for a change in the controlvoltage VC, thereby suppressing a jitter. The oscillation frequency f ofthe VCO is determined by the equation (2). More specifically, theoscillation frequency f can be expressed in the following equation (4).$\begin{matrix}{{\left\lbrack {{Equation}\quad 4} \right\rbrack\quad\quad{f = {\frac{1}{2\pi}L^{- \frac{1}{2}}C^{- \frac{1}{2}}}}}\quad} & (4)\end{matrix}$

Accordingly, df/dVC can be obtained by the following equation (5).$\begin{matrix}{{\left\lbrack {{Equation}\quad 5} \right\rbrack\quad\quad{\frac{\mathbb{d}f}{{\mathbb{d}V}\quad C} = {\frac{1}{2\pi}{L^{- \frac{1}{2}}\left( {- \frac{1}{2}} \right)}C^{- \frac{3}{2}}\frac{\mathbb{d}C}{{\mathbb{d}V}\quad C}}}}\quad} & (5)\end{matrix}$

In the equation (5), it is necessary to reduce dC/dVC (=dCv/dVC) inorder to decrease df/dVC to be referred to as a gain of the VCO.

More specifically, as described above, C in the equation (2) isdetermined by the sum of the variable capacitance Cv and the fixedcapacitance Cf. Therefore, it is possible to decrease a ratio of thevariable capacitance Cv to the fixed capacitance Cf, thereby reducingdf/dVC. Thus, the jitter of the VCO can be reduced.

Accordingly, the fixed capacitance Cf is implemented by the MIM typecapacitance according to the seventeenth embodiment and the variablecapacitance Cv is implemented by the insulated gate type capacitance.Consequently, it is possible to obtain a VCO in which the jitter can bereduced at a desirable oscillation frequency f.

While the insulated gate type capacitance is used as the variablecapacitance Cv and the MIM type capacitance is used as the fixedcapacitance Cf in the application example, a junction type variablecapacitance can also be used for the variable capacitance Cv in place ofthe insulated gate type capacitance.

Moreover, in the case in which a d.c. resistance component can bereduced by a processing such as siliciding, the PIP type capacitance maybe used for the fixed capacitance Cf. The PIP type capacitance is of apolysilicon-insulator-polysilicon type.

<Eighteenth Embodiment>

FIG. 47 is a circuit diagram showing a part of a circuit structure of aVCO according to an eighteenth embodiment of the present invention. Abasic structure is the same as that of the circuit shown in FIG. 46 anddifferent portions will be described below.

One of ends of each of a variable capacitance Cv1 and fixed capacitancesCf1A to Cf1C is connected to a node N3 in parallel, a terminal P2 isconnected to the other end of the variable capacitance Cv1, a terminalP2A is connected to the other end of the fixed capacitance Cf1A (acapacitance value of 0.5 pF), a terminal P2B is connected to the otherend of the fixed capacitance Cf1B (a capacitance value of 1.0 pF), and aterminal P2C is connected to the other end of the fixed capacitance Cf1C(a capacitance value of 2.0 pF). Three fixed capacitances correspondingto the fixed capacitances Cf1A to Cf1C are connected in parallel to anode N4 together with a variable capacitance Cv2 on the node N4 side inthe same manner as the node N3 side, which is not shown in FIG. 47. Inthe terminals P2A to P2C, presence of a connection can be controlleddepending on application (a floating state) of a constant voltage (Vf1Ato Vf1C).

In the eighteenth embodiment, thus, the fixed capacitances Cf1A to Cf1Chaving a plurality of capacitance values with a variation in a size orthe like are connected as the fixed capacitances and a predeterminedvoltage is selectively applied to the terminals P2A to P2C of the fixedcapacitances Cf1A to Cf1C so that a plurality of capacitance values canoptionally be selected. For example, the fixed capacitance value of 0.5pF is obtained if a constant voltage Vf1A is applied to only theterminal P2A to bring the terminals P2B and P2C into the floating state,and a fixed capacitance value of 1.5 pF is obtained if constant voltagesVf1A and Vf1B are applied to the terminals P2A and P2B to bring theterminal P2C into the floating state.

In the eighteenth embodiment, thus, the application of a constantvoltage to the terminals P2A to P2C is optionally selected so that thefixed capacitance value can be selected at an interval of 0.5 pF between0.0 pF and 3.5 pF. Consequently, a central value of an oscillationfrequency f can be changed greatly.

Examples of the fixed capacitances Cf1A to Cf1C include an MIM typecapacitance, a PIP type capacitance, an invariable insulated gate typecapacitance and the like.

<Nineteenth Embodiment>

It is important that a resistance value of a gate resistor should bedecreased in an insulated gate type capacitance. FIG. 48 is a plan viewshowing a gate contact structure of an insulated gate type capacitanceaccording to a nineteenth embodiment of the present invention.

As shown in FIG. 48, a gate electrode 202 provided on a field region 201has a gate contact pad 202 a provided on both ends thereof, and a metalwiring 204 provided around the field region 201 is electricallyconnected to the gate contact pad 202 a on both ends through a gatecontact 203.

In an insulated gate type capacitance C31, moreover, the field region201 and a metal wiring 206 are electrically connected to each otherthrough a plurality of contacts 205. The metal wirings 204 and 206 havesignal propagating portions 204 a and 206 a formed along a virtual lineVL1, respectively. More specifically, an input signal IN to bepropagated to the signal propagating portion 204 a of the metal wiring204 is sent as an output signal OUT from the signal propagating portion206 a of the metal wiring 206.

In the insulated gate type capacitance C31 according to the nineteenthembodiment, thus, the gate contact 203 is provided in the gate contactpad 202 a on both ends of the gate electrode 202, and the gate electrode202 and the metal wiring 204 are electrically connected to each other.Consequently, it is possible to reduce a gate resistance of the gateelectrode 202.

Moreover, since the signal propagating portion 204 a of the metal wiring204 and the signal propagating portion 206 a of the metal wiring 206 areformed along the virtual line VL1, a high frequency signal can be sentlinearly. Consequently, it is possible to obtain a desirable layoutstructure for a high frequency device in which a resistance can bereduced and signal propagation has no loss.

While the signal propagating portion 204 a of the metal wiring 204 isformed in a position indicated as A1 in FIG. 48, it may be formed in aposition indicated as A2 or A3. While the signal propagating portion 206a of the metal wiring 206 is formed in a position indicated as B1 inFIG. 48, it may be formed in a position indicated as B2.

For example, if the signal propagating portion 204 a is formed in theposition indicated as A2 and the signal propagating portion 206 a isformed in the position indicated as B2, the signal propagating portion204 a and the signal propagating portion 206 a are not formed along avirtual line such as the virtual line VL1. Therefore, a transmissionefficiency is decreased and a jitter component of a VCO is increased.Thus, a circuit performance is reduced.

<Twentieth Embodiment>

(First Mode)

FIG. 49 is a plan view showing a first mode of a gate contact structureof an insulated gate type capacitance according to a twentiethembodiment of the present invention. As shown in FIG. 49, a field region207 provided with a rectangular hollow portion 220 on a center andhaving a rectangular peripheral shape is formed and a gate electrode 208is formed with four partial gate electrode portions 208 g uniformlyextended in four directions around a gate contact pad 208 a provided ona hollow portion 220. Gate contact pads 208 b are formed on tips of thefour partial gate electrode portions 208 g, respectively. A sectionalstructure of the insulated gate type capacitance is the same as thesectional structure of the insulated gate type capacitance described ineach of the first to thirteenth embodiments or the like.

One gate contact pad 208 a and four gate contact pads 208 b can beelectrically connected to an aluminum wiring (not shown) or the likethrough one gate contact 209 a and four gate contacts 209 b,respectively.

In an insulated gate type capacitance C32 having such a structure, thepartial gate electrode portion 208 g is extended from the hollow portion220 in four directions so that the gate electrode 208 is formed.Therefore, it is possible to reduce a gate resistance by decreasing agate width Wf defined by a formation width of the partial gate electrodeportion 208 g.

As shown in FIG. 49, moreover, the four partial gate electrode portions208 g are uniformly formed in the four directions (upward, downward,left and right). Consequently, it is possible to form the gate electrode208 in a pattern shape which can be formed comparatively easily withoutan oblique direction.

While only the gate contact pad 208 a may be provided, four gate contactpads 208 b can be added to increase an electrical connecting portion toan aluminum wiring or the like, thereby further reducing the gateresistance.

Moreover, since a body electrode portion can be enlarged in the firstmode, a parasitic resistance in that portion can be decreased.

(Second Mode)

FIG. 50 is a plan view showing a second mode of the gate contactstructure of the insulated gate type capacitance according to thetwentieth embodiment of the present invention. As shown in FIG. 50, afield region 207 provided with a rectangular hollow portion on a centerand having a rectangular peripheral shape is formed and a gate electrode208 uniformly has eight partial gate electrode portions 208 g in eightdirections around a gate contact pad 208 a provided on a hollow portion220. Gate contact pads 208 b and 208 c (a pad in the vicinity of acentral portion on the outer periphery of the field region 207 and a padin a comer portion on the outer periphery of the field region 207) areformed on tips of the partial gate electrode portions 208 g,respectively.

One gate contact pad 208 a, four gate contact pads 208 b and four gatecontact pads 208 c can be electrically connected to an aluminum wiring(not shown) or the like through one gate contact 209 a, four gatecontacts 209 b and four gate contacts 209 c, respectively.

An insulated gate type capacitance C33 having such a structure isprovided with the partial gate electrode portion 208 g extended from thehollow portion 220 in the eight directions. In the same manner as in thefirst mode, therefore, it is possible to further reduce a gateresistance by decreasing individual gate widths (Wf1, Wf2).

While only the gate contact pad 208 a may be provided, eight gatecontact pads 208 b and 208 c in total can be added to increase anelectrical connecting portion to an aluminum wiring or the like, therebyfurther reducing the gate resistance.

Moreover, since a body electrode portion can be enlarged in the secondmode, a parasitic resistance in that portion can be decreased.

(Third Mode)

FIG. 51 is a plan view showing a third mode of the gate contactstructure of the insulated gate type capacitance according to thetwentieth embodiment of the present invention. As shown in FIG. 51, afield region 212 provided with a rectangular hollow portion 221 on fourportions therein and having a rectangular peripheral shape is formed anda gate electrode 213 is formed with four partial gate electrode portions213 g uniformly extended in four directions around a gate contact pad213 a provided on each hollow portion 221. Then, a gate contact pad 213b is formed on a tip of the partial gate electrode portion 213 g whichis provided on the outside of the field region 212.

Four gate contact pads 213 a and eight gate contact pads 213 b can beelectrically connected to an aluminum wiring (not shown) or the likethrough four gate contacts 214 b and eight gate contacts 214 b,respectively.

An insulated gate type capacitance C34 according to the third mode hasan equivalent structure to a structure in which four insulated gate typecapacitances according to the first mode are arranged on a plane invertical and transverse directions. The partial gate electrode portion213 g is extended from each hollow portion 221 in the four directions.By decreasing individual gate widths, therefore, it is possible toreduce a gate resistance depending on the number of the hollow portions221.

While only the four gate contact pads 213 a may be provided, eight gatecontact pads 213 b can be added to increase an electrical connectingportion to an aluminum wiring or the like, thereby further reducing thegate resistance.

Moreover, since a body electrode portion can be enlarged in the thirdmode, a parasitic resistance in that portion can be decreased.

<Others>

In the structures according to the fourteenth to twentieth embodiments,the insulated gate type capacitance can be added to the semiconductordevice according to each of the first to thirteenth embodiments or canbe used for further improving the insulated gate type capacitance of thesemiconductor device according to each of the first to thirteenthembodiments. Thus, various combinations can be applied.

While the invention has been shown and described in detail, theforegoing description is in all aspects illustrative and notrestrictive. It is therefore understood that numerous modifications andvariations can be devised without departing from the scope of theinvention.

1. A semiconductor device comprising: a fixed capacitance formed in asemiconductor substrate and having a capacitance value fixed; and avariable capacitance formed in said semiconductor substrate and capableof variably controlling a capacitance value, wherein said fixed and saidvariable capacitance, are connected to each other in parallel, whereinsaid variable capacitance includes a junction capacitance formed by a PNjunction portion, said semiconductor substrate includes a semiconductorsubstrate of a first conductivity type, and said junction capacitanceincludes; a first semiconductor region of a second conductivity typeprovided in an upper layer portion of said semiconductor substrate, saidfirst semiconductor region having a PN junction portion together with aregion of the first conductivity type of said semiconductor substrate; asecond semiconductor region of the first conductivity type provided insaid upper layer portion of said semiconductor substrate; and anisolating region provided in said upper layer portion of saidsemiconductor substrate and serving to isolate said first and secondsemiconductor regions.
 2. The semiconductor device according to claim 1,wherein said fixed capacitance includes an MIM (Metal Insulator Metal)capacitance.
 3. The semiconductor device according to claim 1, whereinsaid fixed capacitance includes a PIP (Polysilicon InsulatorPolysilicon) capacitance.
 4. The semiconductor device according to claim1, wherein said fixed capacitance includes an insulated gatecapacitance, said insulated gate capacitance including: a gateinsulating film for a capacitance selectively formed on saidsemiconductor substrate; a gate electrode for a capacitance formed onsaid gate insulating film for a capacitance and receiving a fixedelectric potential; and extraction electrode regions formed to interposetherebetween a body region for a capacitance which is provided undersaid gate electrode for a capacitance in a surface of said semiconductorsubstrate.
 5. The semiconductor device according to claim 1, whereinsaid fixed capacitance includes a plurality of partial fixedcapacitances having different capacitance values.
 6. The semiconductordevice according to claim 1, wherein said semiconductor substrateincludes an SOI substrate having a substrate, at least a surface ofwhich is insulative, and an SOI layer provided on said surface of saidsemiconductor substrate, and said isolating region includes a partialisolating region constituted by a partial insulating region provided inan upper layer portion of said SOI layer and an isolating semiconductorregion of the first conductivity type which is a part of said SOI layerpresent in a lower layer portion, said isolating semiconductor regionhaving a PN junction portion together with said first semiconductorregion.
 7. The semiconductor device according to claim 1, furthercomprising: first and second suicide regions formed on surfaces of saidfirst and second semiconductor regions, respectively.
 8. Thesemiconductor device according to claim 1, wherein said fixedcapacitance includes a fixed capacitance having a greater capacitancevalue than a maximum capacitance value of said variable capacitance.